Pumped Storage: Investigating Development of the Elmhurst Quarry Project

Currently being studied for development, the Elmhurst Quarry Pumped Storage Project will take advantage of preexisting resources to bring a renewable energy solution to one of the nation's largest metropolitan areas, Chicago.

By Anthony Charlton and Thomas Haag

Editor's Note: It is hard to quantify the total universe of abandoned rock quarries, but there are more than 1,000 in the National Park system, which encompasses 84 million acres of the total 2.3 billion acres of land in the U.S. These quarries provide infrastructure that could be converted into pumped-storage facilities with less civil work than a greenfield development and minimal environmental impact. The project mentioned in this article is just one example of work under way to tap this resource.

The Elmhurst Quarry Pumped Storage Project (EQPS) is a unique application for pumped storage. The site in the city of Elmhurst, Ill., is just 20 miles from downtown Chicago. EQPS is being developed by Dupage County, Ill., to optimize the value of flood control resources and renewable energy production within one of the nation's largest metropolitan areas. The quarry site is presently being used as a critical flood storage resource.

The project design and location provide a wide range of advantages. Not only is it situated within a major load center, EQPS utilizes an abandoned limestone quarry with both surface and underground mine works that can be used as pumped-storage reservoirs with low environmental impact and cost as compared to other pumped-storage alternatives in the region. The site is also located adjacent to a transmission line. And the addition of a pumped storage component will enhance the existing Salt Creek flood storage operation by providing faster quarry emptying after a flood event.

EQPS has an initial design capacity of 50 MW to 250 MW, with anticipated annual generation of 708.5 GWh. Final project capacity will be determined based on the selected method of project pump and generation cycle operation and long- or short-term energy storage requirements. The upper and lower reservoirs will each have a minimum active capacity of about 7,465 acre feet.

Preliminary engineering has been completed and discussions held with the local utility and others interested in development. The next step is to evaluate the potential list of developers and determine what is needed with regard to funding and a power purchase/sale strategy by the end of 2013. The project is being developed under a Federal Energy Regulatory Commission preliminary permit.

The project site

The site has several important pumped-storage development characteristics:

- Located at a major electric load center;
-Defined as brownfield and suitable for pumped storage development;
- Existing upper and lower reservoirs will decrease cost and time for construction; and
-Transmission access is ideal, with only a 2-mile link to a substation owned by Commonwealth Edison.

Dupage County owns Elmhurst Quarry, and the county's Department of Environmental Concern operates the facility to provide flood control storage for the adjacent Salt Creek. During floods, water from the creek is diverted to the quarry for storage. After the flood peak has passed, the stored water is pumped back to Salt Creek using a pumping plant built in the mid-1990s.

Elmhurst Quarry consists of a surface quarry and an underground mine, with a vertical separation of about 300 feet. The surface quarry operation was initiated in 1890 and continued until 1980, when the dolomite rock source was depleted and operations were transferred to the underground mine. The county took over the land in 1992.

The surface quarry has east and west lobes, separated by a wall about 125 feet wide and 200 feet high. These lobes are connected by a tunnel through the high wall. Depending on the energy storage capacity selected during final design, the pumped-storage project may utilize either or both lobes as the upper reservoir.

The average ground surface elevation across the perimeter of the quarry site is about 680 feet. The average elevation of the bottom of the east lobe is about 480 feet. The bottom elevation of the west lobe varies from about 640 feet at the west end to 520 feet at the east end, near the keyway. The walls of the surface quarry are nearly vertical.

A 25-foot-wide by 17-foot-high declining ramp descends from the floor of the east lobe to provide access to the underground mine. In addition, an 8-foot-diameter steel-lined ventilation shaft extends vertically from the underground mine to the floor of the east lobe. The ramp and shaft were both sealed to isolate the surface quarry for flood storage operation. However, this can be reversed for construction of EQPS.

Determining suitability of the site

The county commissioned initial geotechnical investigation of the surface quarry in the early 1990s. The results indicated the surface quarry is suitable for storage of floodwater; however, final studies will be conducted to determine the potential effects of this change in operations on rock stability on the surface quarry perimeter. These studies will be performed by the selected design team.

The underground portion of the mine extends beneath the Elmhurst Quarry property, including both lobes of the surface quarry, as well as two local roads and Salt Creek. The underground works are about 500 feet below original grade and about 250 feet below the bottom of the surface quarry. A shale layer about 200 feet thick separates the bottom of the surface quarry from the underground mine. A 40-foot-thick layer of dolomite was left in place below the shale layer to serve as the roof for mining operations. Mining was conducted by the room and pillar method, with about 40-foot-square pillars at about 80-foot centers.

The total plan area of the mine is estimated to be 97 acres. It is estimated that the existing mine has a water storage capacity of 12,400 acre-feet, which becomes the ultimate controlling factor in sizing the project. During the design flood event, the active pumped storage water volume of 7,465 acre-feet will be temporarily stored in the underground mine to allow the entire surface quarry to be used for flood storage.

Description of EQPS

The normal maximum operating level of the upper reservoir will be at a height of 667 feet, and the normal minimum operating level will be at 500 feet. The water surface area will vary from 33.9 acres at minimum to 55.1 acres at maximum. At normal maximum operating level, total reservoir storage capacity will be 8,145 acre-feet, and total active capacity will be 7,465 acre-feet, with a total annual production of 708.5 GWh. The average net head will be 435 feet.

The underground mine will be the lower reservoir for the pumped-storage project. The normal maximum level will be at 210 feet elevation and the normal minimum level will be at 84 feet.

A cavern-type powerhouse 80 feet wide by 185 feet long will be excavated 50 feet below the lower reservoir and 700 feet below the surface. The powerhouse will connect to both reservoirs by inlet/outlet structures. Shafts will be excavated for access, ventilation and to carry power conductors to the substation.

A 2-mile-long, 138 kV underground transmission line will provide interconnection to the Commonwealth Edison transmission system.

Inlet/outlet structures

New upper and lower concrete inlet/outlet structures will be constructed on the north side of the east lobe of the surface quarry. The face of the structures will be about 30 feet high, with the width being determined based on plant capacity and operating flow. Approach channels will be excavated within the bottom of the quarry or mine to direct flows to and from the inlet/outlet structure. The inlet/outlet structure construction will include slide gates for dewatering.

Powerhouse cavern

The powerhouse cavern will be sized to accommodate one or more reversible pump-turbine units directly coupled to motor-generators. There is no theoretical maximum number of units, but this is anticipated to be no more than five.

Rock reinforcement for the roof of the powerhouse cavern and other underground chambers will be designed to ensure the existence of a reinforced rock arch capable of withstanding the stresses and potential seepage pressures that may develop. The powerhouse substructure and superstructure construction will include exposed rock surfaces and cast-in-place reinforced concrete. The configuration of these structures will be dictated by final mechanical and electrical equipment arrangements.

Equipment access to the powerhouse cavern will be provided by a shaft about 25 feet in diameter extending from the roof to the ground surface. At the ground surface, the access shaft will be enclosed within the plant access building. This building will house a hoist shaft with a circular bridge crane, plant operations equipment, maintenance facility, power cable shaft, and personnel access shaft with elevator and emergency stair access.

An escape shaft will also exit the powerhouse cavern at an elevation of 57 feet. The shaft will be connected to the gate access tunnel, which would be equipped with watertight bulkhead doors.

Power and tailrace tunnels

The power tunnel will extend about 870 feet between the upper inlet/outlet structure and the pump-turbine inlet valves. The inside diameter of the tunnel is dependent on project capacity and would be lined with concrete or steel.

The tailrace tunnel will extend between the lower inlet/outlet structure and pump-turbine draft tubes. The tailrace tunnel will be about 120 feet long and will be either unlined or lined with concrete.

The proposed hydro development will utilize a quarry that's already in place as part of the pumped storage system. The powerhouse will be excavated below the reservoir.

Substation and transmission

The project substation will be located adjacent to the plant access building and will include 138/10.5 kV generator step-up transformers, a 138 kV/4160 V auxiliary transformer, 138 kV breakers with isolating disconnect switches, associated protective relaying and a line disconnect switch with the underground transmission line termination structure.

An underground 138 kV transmission line will extend about 2 miles from the project substation to an overhead double-circuit 138 kV transmission line owned by Commonwealth Edison, where the interconnection will be made. The tentative transmission line route will be west from the project substation for about 0.7 mile to Villa Avenue. The transmission line will then extend about 1.3 miles in a northerly direction within the Villa Avenue right-of-way to the intersection with the existing transmission lines.

A switchyard will be provided at the existing double circuit overhead line intersection. It will include a cable termination structure for the underground transmission line, common bus and separate breaker with isolating disconnect switches for connection to either line. A substation control building will house the protective relaying equipment.

Project utilization

The county intends EQPS to support the nation's growing installation of intermittent renewable energy resources (such as wind and solar). The county will rely on the project developer to configure and operate the project in conjunction with the site's present use for flood control.

The EQPS project final configuration is anticipated to be utilized in one of the following modes of operation:

-Operate as either a peaking or capacity support plant with storage used on a daily cycle. This is most applicable to support changes in system supply from intermittent renewable resources, such as a wind farm;

- Operate as an energy storage facility to supply energy during extended periods when other renewable resources are not available. In this mode, plant capacity selection will be tied to the expected duration of this resource unavailability; or

- Hybrids of the above options, such as one large unit for short-term peaking operation and one smaller unit for a longer cycle energy storage operation.

The quarry's location outside the city of Chicago gives the project a developmental appeal. Its success could potentially spur additional urban pumped-storage development.

Development status and plan

The county obtained the Federal Energy Regulatory Commission preliminary permit on March 4, 2011.

When the flood control facility was developed in the early 1990s, most of the environmental concerns were identified and addressed. The addition of pumped storage to the facility raises the following issues:

- Transmission access. The 2-mile transmission line will be underground;

- Surface facilities. The county expects the surface facilities to blend with the urban environment and generally be hidden from public view;

- Groundwater impact due to utilization of the mine. Initial consultation indicated that as long as a thorough investigation and treatment design of the underground mine was performed, a 401 water quality permit would be issued; and

- Construction impacts. The project will be constructed in a major urban environment, and mitigation for impacts such as dust, noise and traffic control will need to be addressed.

Proposals for this project will be based on the selected developer providing total development, ownership and operation services, including:

- Completion of all licensing and permitting activities, including the FERC license application and all associated studies and investigations. The county will provide assistance in terms of public, political and regulatory support;

- Design and construction;

- Financing. The county will consider assisting the developer utilizing its municipal financing capability;

- Operation and maintenance; and

- All matters associated with power purchase and sale, transmission interface, and obtaining project revenues.

The county will retain ownership of the land, with a long-term lease the likely contractual arrangement. However, the county will entertain other contractual arrangements.

Conclusion

EQPS is a unique example of the potential for sustainable development of underutilized urban brownfield land. The project has already-developed lower and upper reservoirs and short transmission access to minimize development cost.

- Minimal environmental impacts and licensing that should proceed without major issues; and

- The county and other local municipalities will be supportive of the project, as well as the public in general (the county leadership has consulted with key community leaders and concerned citizens).

For these reasons, the county believes the EQPS project represents a unique opportunity to demonstrate the viability of pumped storage in an urban environment.

Anthony Charlton is director of the Department of Environmental Concerns for Dupage County. Thomas Haag, P.E., is senior project manager/engineer with Black & Veatch and is the company's pumped storage community of practice leader.